INFLUENCE OF EARTHWORMS ON SOIL REACTION. 423 
a feature especially marked with respect to the Molluscan fauna. Atkins * 
has recently published data emphasizing the importance of this factor in the 
distribution of snails. 
It is then clearly essential to know how far earthworms are affected by 
the reaction of the soil. S. H. Hurwitz performed experiments with Allo- 
lobophora fætida (* The Reaction of Earthworms to Acids," Proc. Amer. Acad. 
vol. xlvi. pp. 67-81, 1910), in which the tips of suspended worms were 
dipped into dilute solutions „M, of Hydrochloric, Nitric, Sulphuric, and Acetic 
acids. The time which elapsed, before contraction resulted in the withdrawal 
of the worm from the solution, was determined for a number of individuals, 
and it was found that the reaction time diminished with the increasing dis- 
sociation, thus indicating a considerable sensitiveness to the hydrogen-ion 
concentration. Similar experiments performed by A. T. Sohl (“ Reactions of 
Earthworms to Hydroxylions," Journ. Amer. Physiology, vol. xxxiv. pp. 384- 
404, 1914) on the same species showed a correspondingly diminishing 
2 
“reaction time”? with increasing concentration of Hydroxyl ions. These 
experiments taken together appear to warrant the suggestion that an approxi- 
mation to equality in the concentration ef Hydrogen and Hydroxy] ions is the 
optimum condition for these animals. Arrhenius has performed experiments 
with both Pericheta indica and Lumbricus terrestris, in which soils were 
rendered artificially acid or alkaline. Both species were subjected to a 
range from pH 3 to pH 10, and at the end of a few days live individuals 
remained only in the soils which were very slightly acid or neutral (pH 6- 
pH 7). 
The conditions in all these experiments, however, were clearly very 
artificial, so that observations were made by the writer on natural areas of 
varying reaction, the number of worms being determined both by actual 
counts of the earthworms in a eubit foot of soil, and by enumerating the 
freshly-formed wormeasts on a square yard. The average values and the 
observed range on each of the areas examined is shown in Table VIT. It will 
be noted that broadly there is a diminution in numbers on either side of the 
neutral region (with respect to the diminution at pH 7:0 this is probably 
accidental: cf. below). 
Tagze VII. 
Earthworm Frequency in Natural Soils of varying pH. 
| 
Real acidity (pH) .......... 57 | 6:2 6:4 1:0 T2 14 
| Av. Earthworms l en 1 (32 352 | 93 14 19:0 
per square yard | | 
Observed range...... NNNM O1 (O11 18-60 | 7-18 72-960 | 5-29 
FAT een 0-484 | 15,488 | 170,386 45,012 096,960 | 81,960 
| 
' if 
* Atkins, W. R.G., & M, V. Lebour, in Scient. Proc. Roy. Dublin Soc. xvii. (1925) 
pp. 233-240. 
